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SLO vs. SLA vs. SLI: Understanding the basics of SRE

Any company with a web-based application that experiences frequent outages and poor performance would benefit from investing in Service Reliability Engineering (SRE). SRE strategies can improve the customer experience while reducing operational costs associated with downtime.

But service reliability depends on a trio of fundamental, interrelated SRE concepts that may be confusing to those new to the field. SLO vs. SLA vs. SLI: Which should be your focus?

What is site reliability engineering?

What is SRE, exactly? Put simply, it's a discipline that combines software engineering, operations, and systems reliability principles to ensure services are highly available, reliable, and resilient.

Service reliability engineers are external or internal teams that design incident management software stacks leveraging automated systems to monitor service health and perform operational tasks. These include provisioning resources or deploying new software versions. They work on capacity planning to make sure there's sufficient computing power for scalability needs and strive toward automating response actions so incidents can be quickly resolved without manual intervention from engineers.

SRE teams build internal systems and processes to serve internal customer expectations, i.e., software development or engineering teams. But in so doing, they also keep external customers happy because they help maintain uptime that bolsters development work.

Note that SRE teams aren't DevOps teams, but their relationship is symbiotic. DevOps requires skilled SREs to build, manage, maintain, and optimize a production environment according to best practices. Meanwhile, SREs benefit from the habit of automation that DevOps provides. Both teams need good communication skills for their collaboration to be successful.

Essentially, SRE ensures that services run smoothly 24/7 with minimal manual intervention from engineers. It covers many different types of incidents and how to deal with them. Software companies rely on different types of entities to meet customer expectations, and SRE teams are one of those moving parts that generally support other teams.

What are Service Level Objectives (SLOs)?

Service Level Objectives measure overall service performance. SLOs define the required availability, latency, and errors of a system. They are typically set to achieve customer satisfaction while balancing cost-efficiency goals. SLOs should be measurable, achievable, and relevant to what customers require from the service to meet their needs. These internal objectives should also be discussed with stakeholders so that you can find an appropriate balance between cost/effort and customer expectations on quality/reliability levels for the service.

Let's say you are running a customer service platform, and your SLO is to ensure less than 20 seconds of average response time for all queries coming into the system. You would set up monitoring systems to track this metric in real-time to make sure the results are consistently under 20 seconds.

If this time is exceeded at any point, SREs will take proactive steps, such as scaling out or provisioning more resources, to maintain acceptable quality levels for customers experiencing delays due to a lack of capacity/resources from your side.

What are Service Level Agreements (SLAs)?

A Service Level Agreement is the contractual agreement between a provider and a client regarding the service performance of an SRE team.

An SLA outlines what type of support will be provided, incident response times, the turnaround for fixes/changes made by engineers, and other relevant matters. It also defines potential incentives or penalties for meeting or not meeting these commitments according to predetermined parameters and measurable metrics agreed upon by both parties.

This ensures that all stakeholders clearly understand who is responsible and accountable when it comes to upholding quality standards around reliability and availability.

Say you’re setting up a service contract between your company and a customer. In the SLA, both parties would agree on how many hours of support will be provided in case of system outages or other incidents, what response time the SRE team should adhere to when it comes to fixing issues reported by customers, as well as any monetary penalties if these criteria aren't met for whatever reason (e.g., due to resource shortages from one side).

Creating an agreement beforehand allows both sides involved to know exactly what their responsibilities are and helps define quality standards for reliability and availability to ensure high-quality customer service.

Now, when it comes to agreements, you can expect some measure of involvement from your legal teams. Legally binding agreements between customers/users and service providers may need to be established before addressing the technical aspects of SLOs and SLIs, but they're particularly important in SLAs.

What are Service Level Indicators (SLIs)?

Service Level Indicators are metrics or actual measurements used to track, monitor, and report on an SRE team's performance.

SLIs are usually based on the SLO target set by both parties, as well as other important metrics such as uptime percentage and error rates. They help provide visibility into overall system health (from an operational perspective) so that potential issues can be quickly identified and addressed before they become bigger problems. Ultimately, these indicators allow SRE teams to ensure their service meets the customer expectations indicated through established SLOs.

If, for example, your SLO target is 99% uptime for your service, you might use the SLI metric "availability" to monitor this performance and make sure it stays within acceptable levels that meet or exceed customer expectations. You can track metrics such as page loading times, downtime duration, response time for requests sent over a certain period, and others. This will give you a look into the overall system health, allowing you to quickly identify potential problems and respond proactively (e.g., scaling out/provisioning more resources).

How do these three compare?

So that covers SRE fundamentals, but how do they compare and work together?

Key differences between SLOs, SLAs, and SLIs

The differences between the three lie in the service levels they deal with: objectives, agreements, or metrics or indicators.

  • SLO is a target performance metric set to achieve customer satisfaction while balancing cost efficiency goals.
  • SLA is a contractual commitment between two parties regarding service performance and outlines what type of support will be provided, how long response times should be, and other terms.
  • SLI tracks and reports on the performance against the SLO target. It includes metrics such as uptime percentage or error rates.

Key similarities

SLO, SLA, SLI—they're all related to service performance and provide visibility into overall system health. They are used to ensure services meet customer expectations in terms of availability, reliability, scalability, and overall uptime. Additionally, they help identify potential problems before they snowball by providing insight into how the system is performing against predetermined metrics/goals.

How they work together in SRE

SLO, SLA, and SLI are the three pillars of a successful SRE practice. Each complements the other to provide an effective system that meets customer expectations while balancing cost-efficiency goals.

SLOs set targets for customer satisfaction and cost efficiency goals. SLAs outline how to deal with failure to meet these targets, and SLIs track actual performance against the SLOs so potential issues can be dealt with efficiently. They work together to ensure service reliability.

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Luis Gonzalez
Content Marketing Manager